In accordance with the requirements of the American Bowling Congress ("ABC"), all bowling balls must conform to a limited size range regardless of weight variations, which may be in a range of six to sixteen pounds (2.72 to 7.26 kilograms). Most balls for use by adults have a weight of about sixteen pounds (7.26 kilos). A bowling ball, under ABC requirements, cannot contain metal components, though use of metallic compounds is permitted in the cores. As most manufacturers produce them, the principal differences between bowling balls of different weights are based on variations in the density (specific gravity) of their cores. Thus, the cores in a particular bowling ball construction are often all of the same size and shape, and the weight of the shell encompassing the core is about the same, though the shells may exhibit some weight variation. In virtually every bowling ball there is a balancing weight to compensate for finger holes drilled in the ball shell to accommodate the fingers used by a particular bowler in gripping the ball. The balancing weight is preferably a part of the core, but may be separate from the core.
The mechanics of a bowling ball moving down a lane toward the pins are complex and are not always well understood. As released by a typical high-scoring bowler, the bowling ball exhibits both linear (sliding) velocity, or speed, and rotational velocity. At release, the bowling ball is usually rotating about an axis determined by the bowler, an axis that may be quite different from any of the usually recognized axes of the ball. As the ball moves down the lane toward the pins the initially predominant linear motion tends to decelerate more rapidly, due to frictional engagement with the lane. Rotational movement shows less deceleration, but may change, both in amplitude and in regard to the axis of rotation.
The ABC does not specify limits for moments of inertia for a bowling ball, but does specify permissible maximum values for radii of gyration (RG) about three principal axes X, Y and Z. The Z axis is the "pin" axis of the ball; the X and Y axes are perpendicular to the Z axis and to each other. All three axes intersect at the center of gravity of the ball. ABC specifications also cover the differentials permissible between the RG values of a bowling ball about its axes. These differentials are limited to a maximum measured value of 0.080; there is no specified minimum measured differential value.
In bowling, the angle at which a bowling ball strikes the head pin is an important factor in the effect on the pins. That is why proficient bowlers prefer a ball that consistently describes a curve or "hook" as it approaches the pins. If the hook begins too soon or too late, as the ball moves down the lane toward the pins, the hook effect changes and the results may be quite undesirable or even disastrous.
The factors that affect the hook exhibited by a bowling ball are known, but their inter-relationships are not always fully understood. Most lanes are oiled in the area where the bowling ball first engages the lane; usually, however, the lane area adjacent the bowling pins is not oiled. Friction between the surface of the bowling ball and the lane does not cause the ball to hook, but it does affect the timing and extent of the hooking action. The speed of the ball affects the hook action; if ball speed is increased, the forces governing hook action are reduced. Broadly speaking, the slower the bowling ball rolls the more it will hook, and vice versa. The axis of the initial spin of the bowling ball (the spin created by the way the bowler releases the ball), and the rotational speed of that spin, both affect the hooking action. Indeed, ball rotational speed and the axis of rotation are perhaps the most significant factors affecting hook. Rotational speed, as imparted by the bowler, is not a factor that can be controlled by manipulation of the bowling ball structure; it depends on the bowler. The extent of lane oiling is also beyond control of the ball manufacturer. But the frictional characteristics of the outer surface of the bowling ball and the locations of the axes of the ball, as well as the RG values applicable to those axes, are subject to control by the manufacturer when the bowling ball is made. The present invention is concerned with those factors subject to control at the time of manufacture.
In many bowling ball constructions, the core is essentially spherical. A small balancing weight is provided to compensate for the finger holes, which holes are usually drilled at the time of sale to a particular bowler. The balancing weight may be a part of the core or it may be a separate element. A bowling ball with a symmetrical core has no particular location for a preferred spin axis (PSA); the PSA position is inconsistent and unpredictable. The PSA is likely to shift when the finger holes are drilled; the dynamic characteristics of the ball are still random and unpredictable.
Bowling ball cores of rather unusual configurations have been proposed and used; most seem to be based on empirical determinations or even just plain guesswork. A bowling ball core that is asymmetrical, but may have one "mirror plane", so that the RG values are different for all axes of the ball, is a substantial improvement. Static imbalances and weight voids have less effect on the bowling ball reaction. When the bowling ball is drilled, its PSA shifts only slightly; the ball is more predictable than one which has a truly spherical core. But the PSA is still subject to some change.